An Oracle RAC' s instance but executed in one single node. May be run on different physical servers, not only one (this is common) but a database instance is running only on the same node
simultaneously (* but we'll see later that this is not completely true).

It's more easy to understand seeing this diagram:

In this scheme, we have 3 physical servers containing 5 total instances running on
Oracle RAC One Node . Each node has an OS, so there aren't different virtual machines.

The most notable advantages are:

It enables high availability of a database instance, both server failure, or DB level (for example, VMware only provide hardware-level server). That is, in the example above, a fall or a problem of unavailability of Server 2, would make the database DB-C moved to another physical server automatically.

Allow distribution of physical resources online (Instance Caging). For example, in the example above,
if Server 1 had 4 physical processors, we could allocate 3 to that DB-A instance and only assign 1 to the DB-B instance . Furthermore, it could allocate dynamically without restart, if any instance needs more processor.

In case of manteinance of a physical server (eg, by need of applying patches, it may move its instances dynamically and without loss of service. This feature is done by Oracle Omotion. To avoid loss of service, during brief instants, Oracle RAC One Node runs a new instance on target server, so in that period time there are 2 instances on the same database (like Oracle RAC).

It can upgrade Oracle RAC at any time without service interruption.

As you can see, the storage should be shared.

Like Oracle RAC, Oracle RAC One Node clustering software uses Oracle Clusterware to control clusterized resources in the different nodes.This does, of course, that compatibility with other clustering software (Veritas, Sun Cluster, etc. ..) is not possible.

To sell the product, Oracle insists on the advantages of having n-physical machines Oracle RAC One Node compared to having them in a VMware farm. These advantages can be summarized as follows:

More efficient use of resources, by not using the hypervisor layer.

Easy patches installation (except operating systems and ability to change the server that manage the service)

CPU Dynamic modification without having to reboot.

The licensing is cheaper than Oracle RAC, so this product can be seen as a lower-cost approach, and later upgrade to Oracle RAC: